Webmail is going to require a mailserver like Sendmail or Postfix, a webserver like Apache, and a webmail program like OpenWebmail or Squirrelmail.
First get Apache up and running. Get your Sendmail or Postfix running. Then install and configure your webmail app.
You will also need to setup a way for users to retrieve their mail. There are a couple of ways, POP or IMAP.
It would be best if you can use secure POP and IMAP so your passwords aren't vunerable.
This may help..
http://www.simpaticus.com/linux/howt...ver-HOWTO.html
Each individual application will have it's own documentation. You need to follow them and post your problems or questions.
There is probably no one single good set of instructions available due to the different services you will need to install.